1. Technical Field
The invention relates to semiconductor process chambers. More particularly, the invention relates to establishing optimum gas flow patterns in semiconductor process chambers.
2. Description of the Prior Art
A critical goal when processing semiconductor substrates in a semiconductor process chamber is to deposit a uniform film of a desired material on the substrate surface. However, known substrate deposition processes do not produce optimal film uniformity, for example as indicated by the film refractive index, which is a measure of the uniformity of the thickness and composition of the film. Lack of film uniformity results in a processed substrate on which there is a variation across the substrate in the electrical properties of the individual devices fabricated thereon. It is thought that lack of uniformity in process gas flow patterns across the substrate surface during substrate processing may be one cause of poor film uniformity.
FIG. 1 is a partially sectioned side elevation view of a prior art semiconductor process chamber that is used for such processes as chemical vapor deposition ("CVD").
As shown in FIG. 1, a semiconductor substrate 28 is loaded into a process chamber 20 through an opening 21 by a robot blade 15. A susceptor 30 is adapted to both support and heat the substrate, and to function as an electrode to which an RF voltage is applied to excite a plasma during substrate processing. The susceptor is supported by a central susceptor hub 36, while a susceptor support arm 31 coupled to a susceptor lift mechanism (not shown) supports the susceptor hub.
During substrate loading, the blade 15 inserts a substrate through the chamber opening 21 and positions the substrate directly above the susceptor 30. Two or more lift fingers 39 are positioned below a substrate transfer location. The lift fingers are supported by a lift finger support ring 40 and lift finger support mechanism 41. Once the substrate is properly positioned by the blade within the process chamber, the lift fingers 39 are raised to lift the substrate 28 from the robot blade 15. The blade is then retracted, and the chamber is sealed and pumped down with a vacuum pump to a desired processing pressure.
Once the substrate is positioned on the lift fingers, the susceptor is raised and brought into contact with the substrate. The susceptor then lifts the substrate along a process chamber central axis, as indicated by the numeric designator 25, until the substrate 28 is positioned at a substrate processing location.
During substrate processing, a process gas is pumped into the process chamber through a port 56 to an apertured gas distribution plate 55, also referred to as a shower head, that directs the process gas toward the substrate. The process gas is subjected to a strong electrical field that creates a plasma above the substrate, and produces a gas-phase chemical reaction which enhances substrate film deposition.
A series of arrows, some of which are identified by the numeric designator 37, show process gas flow within the process chamber. As shown in FIGS. 1 and 2, the process gas flows across the substrate surface, through apertures 61 formed in an exhaust baffle 60, and into an exhaust plenum 23 that partially surrounds the susceptor. The exhaust plenum 23 serves to minimize changes in exhaust pressure within the process chamber, and operates in conjunction with the apertured exhaust baffle to control gas extraction from the process chamber in a manner that distributes gas flow evenly across the substrate surface. The gas is then drawn from the exhaust plenum 23 into a vacuum conduit 24 by a vacuum pump (not shown). A close fit is provided between the adjacent edges 35 of the susceptor and the exhaust baffle to prevent process gas from flowing therebetween.
One source of undesirable nonuniformities in the gas flow pattern is chamber hardware which interrupts the symmetry and continuity of the exhaust plenum 23. For example, the substrate insertion opening 21 is located at the same elevation in the process chamber as the exhaust plenum 23, such that the continuity of the exhaust plenum is interrupted at the opening. Additionally, both the lift finger support mechanism 41 and the susceptor lift mechanism are located adjacent to the central core of the processing chamber at the same elevation in the process chamber as the exhaust plenum 23. Thus, the continuity of the exhaust plenum 23 is also interrupted to provide space for the lift finger support mechanism 41 and the susceptor lift mechanism. As a result, the exhaust plenum is typically arc shaped, having two ends that define a gap therebetween, such that the exhaust plenum does not completely surround the substrate, and such that it cannot draw process gas from the process chamber uniformly about the entire circumference of the substrate.
The prior art exhaust baffle 60, shown in FIG. 2, has a six-sided outer edge 64, and includes a circular ring portion 63 that has a plurality of apertures formed therethrough to allow process gas to flow to the exhaust plenum below. Two sets of apertures 66, 68 are positioned opposite each other on the exhaust baffle ring. The apertures are typically positioned equidistant from the center of the processing chamber. A third set of apertures 67 is positioned intermediate the other two sets of apertures on one side of the exhaust baffle ring.
The dashed arrows, identified in FIG. 2 by the numeric designator 69, show an expected gas flow path across the substrate surface to the apertures near the edge of the exhaust baffle 60 for gas introduced into the process chamber near the center of the gas distribution plate. As shown in the figure, process gas flow across the substrate surface to the apertures 66, 67, 68 leaves several arc sections of the substrate with little process gas cross flow, and only minimal exposure to gas molecules passing from adjacent areas. Although the entire substrate surface has some exposure to process gas, because the gas is pumped directly to the substrate surface from the gas distribution plate 55, the failure to distribute the process gas flow uniformly across the substrate surface is a significant cause of nonuniform film deposition on the substrate surface.